Flash memory is an electronic component used as a computer storage medium, typically comprising a solid-state memory device. NAND-type flash memory may be written and read in blocks and/or pages. It is typically used in memory cards, USB flash drives, and solid-state drives for general storage and transfer of data. NOR-type flash memory may allow writing and reading a single machine word and/or byte. The NOR-based flash devices may require longer erase and/or write times, but provide full address and data buses allowing random access to any memory location. NOR-based devices may be more appropriate for program code that is rarely updated, e.g., a computer BIOS or component firmware.
Flash memory stores data in an array of memory cells comprising floating-gate transistors. Each memory cell may include two gates, a control gate and a floating gate insulated by an oxide layer. FIG. 1 shows a prior art flash memory cell 1 comprising two select gates 10, a control gate 20, a source junction 30 under the control gate 20, a drain junction 40, and a floating gate 50. There is a layer of oxide surrounding the various components. The floating gate 50 can be connected to the source junction 30 by the control gate 20, sometimes referred to as an erase gate. Floating gate 50 can be connected to the drain junction 40 by select gate 10, sometimes referred to as a word line device. The various dimensions of each feature are limited by the manufacturing processes used. Of course, the smaller the dimensions are, the more memory cells 1 may be arrayed in the same size chip and/or device.